The application of a continuous positive airway pressure (CPAP) to a patient's respiratory system is a frequently used therapy for treating sleep disorders such as obstructive sleep apnea. Typically, positive air pressure is applied to the patient's airway through either a nasal mask or a nasal cannula. Apnea is commonly caused by the upper airway being blocked or collapsing during sleep. With CPAP, an inspiratory positive airway pressure (IPAP) applied to the patient is set to a pressure which expands or inflates the airway sufficiently to prevent its blockage during inspiration. The lowest effective IPAP pressure for a patient may be determined through titration in a sleep clinic or by using CPAP apparatus which is programmed to automatically adjusts the applied pressure up and down in response to the occurrence and the absence of sleep events until the events cease. The pressure is increased and decreased while monitoring for abnormal sleep events until the lowest effective IPAP pressure is determined.
CPAP therapy is effective only so long as the patient complies with the prescribed therapy. As many as 20% to 40% of the patients for which CPAP therapy is prescribed fail to adhere to the prescribed therapy due to discomfort. The most frequent source of discomfort is with the nasal mask. A second source of discomfort can result from the additional work of breathing against the applied CPAP pressure. This is particularly noticeable when the CPAP pressure is first applied while the patient is still awake. Inspiration is not a problem with a positive airway pressure. However, there is additional effort in exhaling against the positive airway pressure. One solution for reducing discomfort has been to provide the CPAP apparatus with a soft start. When the apparatus is initially turned on, a relative low positive pressure is applied to the patient's airway. This pressure is below the prescribed pressure necessary for preventing abnormal respiratory events. However, abnormal respiratory events are generally not a problem while the patient is awake. Over a programmed period of time such as 20 or 30 minutes, the applied pressure is gradually increased or ramped up to the prescribed therapeutic pressure. Some CPAP apparatus provides a constant low pressure for a period of time prior to increasing to the prescribed therapeutic pressure. The soft start helps the patient to fall asleep before the pressure is increased to the therapeutic level.
For standard CPAP apparatus, the IPAP pressure and the expiratory positive airway pressure (EPAP) are substantially the same. Another method for reducing discomfort from exhaling against the therapeutic pressure is to provide bilevel CPAP therapy. For obstructive sleep apnea, most airway blockages occur only during inspiration. In a bilevel system, the CPAP apparatus is controlled to increase the applied IPAP pressure to the prescribed therapeutic level in response to sensing the beginning of inspiration and to decrease the applied EPAP pressure to a lower, more comfortable level in response to sensing the beginning of expiration. This reduces the effort and discomfort of exhaling against the prescribed pressure. Bilevel CPAP therapy is particularly useful for patients who require a high IPAP pressure for preventing abnormal sleep events. However, for some patients, bilevel CPAP therapy may not be as effective as applying a constant therapeutic pressure to the patient.